1. Field of The Invention
This invention relates to communication systems and, more particularly, to an OFDM (Orthogonal Frequency Division Multiplexing) transmitter system that uses complementary codes.
2. Description of Related Art
In general, orthogonal frequency multiplexing (OFDM) is a block-oriented modulation scheme that maps N data symbols into N orthogonal carriers separated by a distance of 1/T, where T is the block period. As such, multi-carrier transmission systems use OFDM modulation to send data bits in parallel over multiple, adjacent carriers (also called tones or bins). An important advantage of multi-carrier transmission is that inter-symbol interference due to signal dispersion (or delay spread) in the transmission channel can be reduced or even eliminated by inserting a guard time interval between the transmission of subsequent symbols, thus avoiding an equalizer as required in single carrier systems. The guard time allows delayed copies of each symbol, arriving at the receiver after the initial signal, to die out before the succeeding symbol is received.
An OFDM modulator converts a serial data stream into a block of N complex carriers at a rate of f.sub.block =f.sub.s /N where f.sub.s is the data rate of the serial bit stream. The sum of the individual carriers, of which both phase and amplitude can be modulated, correspond to a time domain wave form that can be generated using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) is a well-known efficient implementation of the IDFT that performs an N-point IDFT transform.
The guard time is inserted into the output of the IDFT and filled in with a copy of the symbol (called a cyclic prefix) to preserve the orthogonality between carriers. The resulting wave form is then convolved with a time window function that provides a gradual ramp-up and down of the symbols in the time domain to ensure a sharp spectrum cut-off in the frequency domain. The latter is particularly important for wireless application where spectrum rules restrict interference due to out-of-band pollution.
A drawback of the OFDM technique is that the peak-to-average power (PAP) ratio of the signal increases as the number of carriers is increased. The increase in the PAP ratio due to the increased number of carriers therefore requires linearity of the transmission. In a wireless channel, for example, highly linear power amplifiers can be used to deal with the PAP ratio problem. These linear power amplifiers, however, are not very power efficient, and the power efficiency issue is a drawback for mobile, portable applications where the power amplifiers are likely to be powered by more costly batteries of limited capacity.
Thus, a need exists for an OFDM transmitter which can efficiently and cost effectively reduce the PAP ratio of transmission signals to provide the benefits of OFDM while reducing the above problems.